Expandable reamers are typically employed for enlarging subterranean borehole. Conventionally in drilling oil, gas, and geothermal wells, casing is installed and cemented to prevent the well bore walls from caving into the subterranean borehole while providing requisite shoring for subsequent drilling operation to achieve greater depths. Casing is also conventionally installed to isolate different formations, to prevent crossflow of formation fluids, and to enable control of formation fluid and pressure as the borehole is drilled. To increase the depth of a previously drilled borehole, new casing is laid within and extended below the previous casing. While adding additional casing allows a borehole to reach greater depths, it has the disadvantage of narrowing the borehole. Narrowing the borehole restricts the diameter of any subsequent sections of the well because the drill bit and any further casing must pass through the existing casing. As reductions in the borehole diameter are undesirable because they limit the production flow rate of oil and gas through the borehole, it is often desirable to enlarge a subterranean borehole to provide a larger borehole diameter for installing additional casing beyond previously installed casing as well as to enable better production flow rates of hydrocarbons through the borehole.
A variety of approaches have been employed for enlarging a borehole diameter. One conventional approach used to enlarge a subterranean borehole includes using eccentric and bi-center bits. For example, an eccentric bit with a laterally extended or enlarged cutting portion is rotated about its axis to produce an enlarged borehole diameter. An example of an eccentric bit is disclosed in U.S. Pat. No. 4,635,738, assigned to the assignee of the present invention. A bi-center bit assembly employs two longitudinally superimposed bit sections with laterally offset axes, which when rotated produce an enlarged borehole diameter. An example of a bi-center bit is disclosed in U.S. Pat. No. 5,957,223, which is also assigned to the assignee of the present invention.
Another conventional approach used to enlarge a subterranean borehole includes employing an extended bottom-hole assembly with a pilot drill bit at the distal end thereof and a reamer assembly some distance above. This arrangement permits the use of any standard rotary drill bit type, be it a rock bit or a drag bit, as the pilot bit, and the extended nature of the assembly permits greater flexibility when passing through tight spots in the borehole as well as the opportunity to effectively stabilize the pilot drill bit so that the pilot hole and the following reamer will traverse the path intended for the borehole. This aspect of an extended bottom-hole assembly is particularly significant in directional drilling. The assignee of the present invention has, to this end, designed as reaming structures so called “reamer wings,” which generally comprise a tubular body having a fishing neck with a threaded connection at the top thereof and a tong die surface at the bottom thereof also with a threaded connection. U.S. Pat. Nos. 5,497,842 and 5,495,899, both assigned to the assignee of the present invention, disclose reaming structures including reamer wings. The upper midportion of the reamer wing tool includes one or more longitudinally extending blades projecting generally radially outwardly from the tubular body, the outer edges of the blades carrying PDC cutting elements.
As mentioned above, conventional expandable reamers may be used to enlarge a subterranean borehole and may include blades pivotably or hingedly affixed to a tubular body and actuated by way of a piston disposed therein as disclosed by U.S. Pat. No. 5,402,856 to Warren. In addition, U.S. Pat. No. 6,360,831 to Åkesson et al. discloses a conventional borehole opener comprising a body equipped with at least two hole opening arms having cutting means that may be moved from a position of rest in the body to an active position by exposure to pressure of the drilling fluid flowing through the body. The blades in these reamers are initially retracted to permit the tool to be run through the borehole on a drill string and once the tool has passed beyond the end of the casing, the blades are extended so the bore diameter may be increased below the casing.
The blades of conventional expandable reamers have been sized to minimize a clearance between themselves and the tubular body in order to prevent any drilling mud and earth fragments from becoming lodged in the clearance and binding the blade against the tubular body. The blades of these conventional expandable reamers utilize pressure from inside the tool to apply force radially outward against pistons which move the blades, carrying cutting elements, laterally outward. It is felt by some that the nature of the conventional reamers allows misaligned forces to cock and jam the pistons and blades, preventing the springs from retracting the blades laterally inward. Also, designs of these conventional expandable reamer assemblies fail to help blade retraction when jammed and pulled upward against the borehole casing. Furthermore, some conventional hydraulically actuated reamers utilize expensive seals disposed around a very complex shaped and expensive piston, or blade, carrying cutting elements. In order to prevent cocking, some conventional reamers are designed having the piston shaped oddly in order to try to avoid the supposed cocking, requiring matching, complex seal configurations. These seals are feared to possibly leak after extended usage.
Other conventional reamers require very close tolerances (such as six-thousandths of an inch (0.006″) in some areas) around the pistons or blades. Testing suggests that this may be a major contributor to the problem of the piston failing to retract the blades back into the tool, due to binding caused by particulate-laden drilling mud.
Notwithstanding the various prior approaches to drill and/or ream a larger diameter borehole below a smaller diameter borehole, the need exists for improved apparatus and methods for doing so. For instance, bi-center and reamer wing assemblies are limited in the sense that the pass-through diameter of such tools is nonadjustable and limited by the reaming diameter. Furthermore, conventional bi-center and eccentric bits may have the tendency to wobble and deviate from the path intended for the borehole. Conventional expandable reaming assemblies, while sometimes more stable than bi-center and eccentric bits, may be subject to damage when passing through a smaller diameter borehole or casing section, may be prematurely actuated, and may present difficulties in removal from the borehole after actuation.
Accordingly, there is an ongoing desire to improve or extend performance of an expandable reamer apparatus regardless of the subterranean formation type being drilled. There is a further desire to provide a reamer apparatus that provides failsafe blade retraction, is robustly designed with conventional seal or sleeve configurations, and may not require sensitive tolerances between moving parts.